I'll be running a one-shot session later this month. There will be five players: three with previous D&D experience, and two entirely new to the game. I'm a Gen Xer (early 50s); the players are all millenials (mid-20s) - my daughter and her friends.  :) I'll be using Dragon of Icespire Peak, with some significant customizations, for the adventure.

This weekend, via Zoom, we'll be doing character creation and session zero. I've never done a deliberate session zero before (I DMed 1E up until 2017 with a group who all knew each other really well, and when I started 5E with a new group, I frankly wasn't aware of the idea of a S0 until after I started DMing for them).

I'm wondering what should be covered, other than character creation, given that it's a one-shot (meaning there isn't an ongoing campaign to talk about expectations or goals) and it's a mixed of experienced and new players. 

I do have a few house rules I could cover, but I'm concerned about the new players feeling overwhelmed.

Any suggestions?

Hello.  First, good luck!  It will be fun, I'm sure. I'm going to make a suggestion just for putting an idea out there.  You, of course, know what's best for your crew.

I suggest that you don't do a Session 0 for a one-shot for newbies.  I suggest that you (and maybe the ones with a little experience) make pregens for the newbies.  This gets them into the game quickly without lots of understanding things that might not even come into play in your one-session adventure.  Sure, they could say a little about their motivation or history at the beginning of the adventure.  That could be fun.  Otherwise, they might spend 4 hours of Session 0 setting up a fighter, only to realize that they don't like playing a fighter.  If they spend that much time on a character, then they'll have some desire to follow up with a longer campaign, too, which you might not be planning for.

If you hit them with pregens and get them interested in the game, they can spend their first session learning how to have fun, and maybe they can get a better idea of what they like.  Do they like roleplaying or combat more?  Do AC and skill modifiers make sense now?  At that point, they might be ready for a full-out campaign with a Session 0 in which they can react to house rules or decisions on gore and violence.

I think a Session 0 is important for longer games.  I think with total newbies, getting them into fun as soon as possible is more important.

Hmm. I actually thought about doing pregens, and letting them pick. However, ultimately, they're making the same decision they would've made if they'd rolled dice (or gone with the standard array). And I want to use this session to help the newbies understand that basically everything their character attempts stems from the ability scores.

Note that my daughter and her friends are geographically separated from me; the one-shot will be in person, as I'm visiting. But there's no chance of an ongoing campaign with this group, unless it's online, and I've not cozened up to online playing as of yet.

I had a pregen for my first game and I concur that it was a far better way to get into the game. If you learn the game in order, the first part is akin to homework and then the second part - if you make it that far - is akin to the actual fun!

Making characters is fun now, but if I had tried to do it without any knowledge of what anything means - what's loading, what does finesse mean, which of my abilities marries with these weapons, which don't, and so on - it would have been overwhelming and I can guarantee I'd have spent most of the session finding out the mistakes I made whilst making my character, which is not fun!

As it was, I was handed a sheet for the class I chose (Barbarian) and given a rundown of what I can do, a quick-reference sheet for actions, and we had at it! and it was so much fun, I came away thinking "wow, if it was that good with a generic character, how cool will it be with my own character?" rather than thinking "wow, I got so much stuff wrong whilst making my character, that sucked!".

I'd recommend perhaps session 0 be used to run them through a mock combat & puzzle routine, with basic character sheets, so they get the gist of what to do without feeling like it's impacting the actual game. Just half an hour or an hour of it, to help them feel like they know what they're doing.

As for the content of session 0, I'd make sure that you cover any homebrew rules - not necessarily in detail (though it couldn't hurt to prepare a summary on paper for them) but so thye know "hey guys, I've got my own rules for fall damage this game, so if you're planning on doing anything involving it, make sure you check with me so you know what you're letting yourself in for!". Though for new players, I'd avoid homebrew rules and stick to hombrew puzzles/monsters!